Outlook OST Size vs. Exchange Mailbox Size

You may (or may not) have noticed that the local cached mailbox for Outlook, also known as the OST file, is larger than the Exchange mailbox size. There is no shortage of misinformation about this topic, some of it incorrectly attributing this to corruption1. For example, the mailbox seen here in Exchange is only 132 MBs:However, on the workstation it is more than double at 321+MBs:There is nothing really wrong with the OST file and this difference is expected. The methods by which Exchange and Outlook compress this information is different; we’ll just say that Exchange is more efficient at this.

Also note that the OST does not actually store information in the entire size of the file; there can be a lot of empty space in an OST. As you work with Outlook and, say, delete items, this is not necessarily reflected in the OST file, i.e. the OST is not 10MBs smaller if you just deleted 10MBs worth of emails.

To reduce the size of the OST without creating a new one, Compact the OST. In the example above, I asked the mailbox owner to compact his OST and after a couple minutes the OST size was reduced to just under 250MBs from 325MBs. Keep this in mind when troubleshooting performance issues with large mailboxes and OSTs in Outlook if you enforce cached mode.

Other things worth noting:

ANSI formatted OSTs and PSTs (previous to Outlook 2003) are limited to 2GBs and with Outlook 2003/2007 UNICODE format, this is now 20GB2. In Outlook 2010, the maximum size is 50GBs.

When you are troubleshooting issues where the local mailbox may be the problem, it is not enough to disable cached mode. You also need to disable offline folder use, otherwise the OST is still being connected to by Outlook.

Remember, when done, put the user back in cached mode.

If you believe an OST or PST is corrupt, ran a scan of the file using the scanpst.exe or scanost.exe located in C:\Program Files\Common Files\System\MSMAPI\1033. See MS KB article here for further details: http://support.microsoft.com/kb/287497

1When dealing with OST corruption, you are more likely to see an MS Office error of faulting module pointing to, for example, mspst32.dll.

2This 20GB size limitation for UNICODE can actually be modified via the registry to a larger size. However, for performance reasons, this is not recommended. The same is not true for the ANSI format; to avoid corruption a protection mechanism will never allow an ANSI PST or OST to grow bigger than 2GBs. To work around this, convert it to UNICODE. See this MS KB article for further details:http://support.microsoft.com/kb/832925